- Flow Directions (0-35 S, 5-60 E)
- Flow accumulations (0-35 S, 5-60 E)
- Drainage Basins
- Streams

- Hydrologically correct DEM (0-35 S, 5-60 E)
- Shaded Relief representation of the DEM (0-35 S, 5-60 E)
- Slope (0-35 S, 5-60 E)
- Aspect (0-35 S, 5-60 E)
- Flow Directions (0-35 S, 5-60 E)
- Flow accumulations (0-35 S, 5-60 E)
- Drainage Basins
- Streams

HYDRO1k is a geographic database aimed at providing comprehensive and consistent global coverage of topographically derived data sets, including streams, drainage basins and ancillary layers derived from digital elevation models. Developed from the USGS' recently released 30 arc-second digital elevation model (DEM) of the world (GTOPO30), HYDRO1k provides a suite of geo-referenced data sets, both raster and vector, which will be of value for all users who need to organize, evaluate, or process hydrologic information on a continental scale.

Developed by the U.S. Geological Survey's EROS Data Center with assistance from UNEP/GRID Sioux Falls, the HYDRO1k project's goal is to provide to users, on a continent by continent basis, hydrologically correct DEMs along with ancillary data sets usable in continental-scale modeling and analysis. Eight derivative data sets, six raster and two vector, are being extracted from GTOPO30 for each continent. The six raster data sets are:

- Hydrologically correct DEM,
- Shaded Relief representation of the DEM,
- Slope,
- Aspect,
- Flow Directions,
- Flow accumulations.

The hydrologically correct DEM forms the basis of all the additional HYDRO1k data sets. This DEM has been processed to fill all spurious sinks, while maintaining sinks that are natural occurrences in the landscape. This time-consuming process yields a DEM, which will properly transport water across its surface.

The shaded relief representation of the DEM was generated using ARC/INFO's Slope-Aspect Index (SAI function) with a vertical exaggeration of 15. The values of the Shaded Relief representation vary from 0 to 255. An example colormap file using an angle of illumination of 315 ø from north and 60ø from the horizon (az315at60) has been used to display the image at right.

The slope data set describes the maximum change in the elevations between each cell and its eight neighbors. The slope is expressed in integer degrees of slope between 0 and 90. The slope image displayed here and available for distribution was created using ARC/INFO's slope function.

The aspect data set describes the direction of maximum rate of change in the elevations between each cell and its eight neighbors. It can essentially be thought of as the slope direction. It is measured in positive integer degrees from 0 to 360, measured clockwise from north. Aspects of cells of zero slope (flat areas) are assigned values of -1.

The flow direction data set, like all of the other raster data sets, has been derived from the hydrologically correct DEM. It defines the direction of flow from each cell to its steepest down-slope neighbor. Values of flow direction vary from 1 to 255. Defined flow directions follow the convention adopted by ARC/INFO's flow direction implementation:

32 | 64 | 128 |

16 | 1 | |

8 | 4 | 2 |

The flow accumulation data set defines the accumulated flow to each cell. Accumulated flow is determined by summing the number of cells that flow into the target cell. Direction of flow is determined from the flow direction data set. Since the cell size of the HYDRO1k data set is 1 km, the flow accumulation value translates directly into drainage areas in square kilometers. Values range from 0 at topographic highs to very large numbers (on the order of millions of cells) at the mouths of large rivers.

The drainage basins distributed with the HYDRO1k data set suite are derived using the vector stream networks along with the flow direction data set. Each polygon in the basin data set has been tagged with a Pfafstetter code uniquely identifying each sub-basin. The five or six digit Pfafstetter codes assigned to each basin carry basin linkage information. This permits determination of basin interconnectedness through simple examination of the Pfafstetter code. Techniques for aggregating and disaggregating the subbasins are described. (See Verdin & Verdin).

The stream network data set is derived by thresholding the flow accumulation data set at 1000 km2. The resulting raster data set is then processed to identify the stream links and each stream link tagged with its flow accumulation value. This results in a vector stream network which carries the DEM derived flow accumulation values as an attribute. This vector data set is available in an ARC/INFO export format.

The HYDRO1k data sets are being staged for Internet distribution, as they become available, continent by continent. To date (October 1997), North America and Africa are complete and available for distribution.